This invention relates to an oxygen measuring electrode assembly having an improved structure which is useful for transcutaneous measurement of oxygen partial pressure in an arterial blood oxygen. The transcutaneous arterial blood oxygen measuring electrode assembly (hereafter abbreviated as "transcutaneous arterial oxygen electrode") is a mechanism for detecting the oxygen partial pressure in the arterial blood of a person to be examined from his skin without directly examining the blood; therefore, the measurement is carried out non-invasively or without damage to any part of his body. Accordingly, this electrode assembly can play an important part in oxygen control of patients, especially newborn infants. Upon application of the electrode assembly of this type onto the skin of a person to be examined, the oxygen in the subcutaneous tissue is diffused through the skin and reaches the noble metal cathode through the electrode membrane, where the oxygen is reduced to provide electrolytic current. Therefore, the oxygen partial pressure value in the tissue can be detected from the value of this electrolytic current. In this case, if the skin in contact with the electrode through the electrode membrane is heated at a temperature as high as possible within a temperature in which the skin is not burned, the subcutaneous tissue adjacent to the electrode is locally arterialized. Accordingly, the oxygen partial pressure of the tissue measured by this method is close to the oxygen partial pressure of arterial blood oxygen. The extent of closeness of the above two values, the transcutaneously measured value and the true arterial value, depends largely on the extent of arterialization of the subcutaneous tissue and partly on the properties of the electrode membrane, shape and size of the cathode and other structures of the electrode assembly. There are two types of transcutaneous arterial oxygen electrode assemblies known in the art. However, each of the electrode assemblies has inherent defects in the electrode structure thereof, especially in the heating mechanism of the skin, size and shape of the cathode and suporting mechanism of the electrode membrane.
In a transcutaneous arterial oxygen electrode assembly of a known first prior art, the cathode is a thin platinum wire whose diameter is on the order of 0.015 mm, and this cathode together with a silver anode arranged in association therewith is covered with an oxygen permeable membrane (polytetrafluoroethylene film 12 microns in thickness) supporting an electrolyte solution on the surfaces of the both electrodes. This electrode membrane is fixed to the electrode supporting body with a rubber "O" ring. In the case of this electrode assembly, heating of the skin is carried out by the silver anode which is heated at a constant temperature (43.degree. C. or 44.degree. C.). In this heating mechanism, arterialization of the subcutaneous tissue is not sufficient, because the heated area of skin is limited by the small surface of the anode. In addition, the signal-to-noise ratio (S/N ratio) in this electrode assembly is very low because the electrodes are extremely small. Furthermore, as the planar membrane is fixed with the "O" ring, the membrane is liable to get crinkled at the fixed portion, therefore it is difficult for the electrode surfaces to be uniformly in contact with the membrane, and accordingly the electrode activity is unstable.
A transcutaneous arterial oxygen electrode assembly of a known second prior art is available on the market. In this electrode assembly, a relatively large (3 mm in diameter) gold cathode is employed, and the cathode and the anode are covered with an electrode membrane (polyester film 6 microns in thickness) which is fixed to the electrode holder with a sleeve. In this electrode assembly, heating of the skin is carried out by the gold cathode which is kept at a constant temperature (42.degree. or 44.degree. C.). In this heating mechanism, arterialization of subcutaneous tissue is not sufficient for the same reasons as described in the case of the first prior art. Although the cathode has a relatively large size (3 mm in diameter) in this second prior art, the size is still too small for the purpose of sufficient arterialization. The second significant drawback of this electrode assembly is that, as the cathode surface must be made large for heating and the amount of oxygen consumption thereof becomes large, the measured oxygen value is measured much less than the actual arterial value. In order to reduce the above drawback. Use of a membrane of very low oxygen permeability has been tried for the electrode membrane. However, this method results in a slow response of the electrode. Accompanying the large oxygen reduction on the cathode, the electrolyte is excessively consumed and the drift of the sensitivity of the electrode becomes considerably large. Furthermore, the peripheral portion of the cathode is different in reactivity from the central portion because these two portions are different from each other in distance from the anode and in supply of ionic components of the electrolyte.
Similarly as in the first prior art described above, the second prior art apparatus also has the drawback that the electrode activity is varied with the change of the contact pressure from the skin, and results in inaccurate measurement, because contact between the membrane and the electrode surface is unstable.